1. Field of the Invention
The present invention relates generally to an inkjet printer and inkjet head that perform printing by ejecting ink onto a recording medium. More particularly, the invention relates to a technique of accelerating a traveling speed of an ink droplet that is ejected.
2. Description of the Related Art
In recent years, inkjet printers that print on a recording medium prevail for personal use and business use. Inkjet heads equipped in such inkjet printers are generally classified into a continuous type and an on-demand type. The continuous type is one in which printing is performed such that, while ink droplet is continuously ejected from a nozzle of the inkjet head, the trajectory of the ink droplet in flight is controlled so as to cause only ink droplets required for printing to land on a recording medium according to the print signals. The on-demand type is one in which printing is performed causing an ink droplet to eject from a nozzle of the inkjet head and deposit on a recording medium at only a required time according to the print signal.
In such inkjet printers, to gain high print-quality, a high accuracy on landing positions of ink droplets onto a recording medium has been demanded.
The print quality is likely subject to a traveling speed of an ink droplet ejected from a nozzle. In normal driving operations carried out in inkjet printers equipped with an on-demand type inkjet head, an initial flight rate of an ink droplet ejected is in the order of some 10 meter/second. At this traveling speed, if a distance between the inkjet head and a recording medium exceeds several millimeters, the resulted print quality likely becomes degraded. This is because ink ejection at such a low speed makes the flight trajectory of the ink droplet unstable due to an air flow or other factors encountered in the course of the ink travel, resulting in irregular landing positioning of the ink droplets on the recording medium. To avoid this problem, the distance between the nozzles and a recording medium needs to be maintained within a predetermined limit. This narrows down the freedom of design of the inkjet printers. On the other hand, in the continuous type, the traveling speed of an inkjet droplet is greater than that of the on-demand type. However, because the distance between the nozzle and a recording medium cannot be shortened due to the structural restriction of this print head, the traveling distance of the ink droplets ejected from the nozzle becomes long, and thus, the landing positions of the ink droplets become irregular.
As one method for solving the above problems, for example, Japanese laid-open patent publication No. 2004-261998 discloses a structure, in which an ultrasound transducer 14 that imparts a vibration force in the direction of ejection of an ink droplet 51 to a print head 31, is provided so as to superimpose the vibration force of the above-mentioned ultrasound transducer 14 (horn 33) of a component in the same direction as the ink flying direction onto the ejection force for the ink droplet 51 ejected from a given nozzle of the print head 31 so that a velocity S2 by the vibration force is superimposed onto a velocity S1 by the ejection force of the ink droplet (Refer to FIG. 4 in the patent publication).
However, this method involves a problem that there needs a complicated control circuit for superimposing the vibration force in the direction of ejection of an ink droplet onto a body of the inkjet head at the same timing as the ink ejection.